Anti-theft systems for motor vehicles are known that are equipped with electrical contacts which actuate the vehicle horn when an attempt is made to open the door. These known anti-theft systems are, however, only effective when unauthorized persons attempt to enter the vehicle. They do not respond upon removal of vehicle parts, particularly road wheels, which are accessible from the outside.
Anti-theft systems are also known which become effective when the vehicle is shaken or rocked. Although these known anti-theft systems set off the alarm when an unauthorized person interferes with the outside of the vehicle, they are very sensitive and also react to external influences that have nothing to do with attempts at theft, for example, when a heavy fast-moving vehicle passes close by, or when strong gusts of wind shake or rock the parked vehicle.